1. Field of the Invention
The present invention relates in general to cardiac monitoring systems, and it more particularly relates to an inventive method and apparatus for data compression in an electro-cardiograph (ECG) monitoring system.
2. Background Information
ECG monitoring systems, such as the portable real-time solid-state Holter monitors, have inherent limitaions in the memory capacity for storing ECG data. These limitations are generally due to constraints on the power consumption and size of the portable monitor, as well as the increased cost of an enlarged memory capacity.
Such limitations render data compression of the ECG signal almost mandatory. However, data compression may require substantial processing time, and may introduce significant distortion.
Several conventional methods for the compression of ECG data have attempted to compensate for the rate of data compression and signal distortion caused thereby. These methods can be classified in two general categories. The first category covers the noiseless or non-distorting compression; and the second category covers the irreversible or distoring compression.
The non-distorting compression mode causes the data or signal to be compressed in such a way that the original data can be reconstructed from the compressed data without losing information. Such non-distorting compression modes are exemplified by the Huffman coding method and the Lempel-Ziv method, as described respectively in the following articles: Huffman, D. A. A method for the construction of minimum-redundancy codes, Proc. IEEE 40: 1098-1101. 1952; and Ziv, J. and Lempel, A. A universal algorithm for sequential data compression, IEEE Trans. Inform. Theory IT-23 pp. 337-343. 1977.
Due to the limitations in the memory capacity of the portable ECG monitoring system, the non-distorting ECG compression method by itself does not generally provide sufficient compression to store a full day (24 hours) of ECG data in the designated memory. Wherefore, distorting compression methods have been used to compensate for the limited memory capacity at the expense of the overall fidelity of the ECG monitoring system.
The distorting compression methods result in the of loss of some of the original ECG information. These methods are exemplified by the AZTEC preprocessing software program, and the SAPA or fan method, as respectively described in the following articles: Cox, J. R., Nolle, F. M., Fozzard, H. A., and Oliver, G. C. Jr. AZTEC, a pre-processing program for real-time ECG rythm analysis, IEEE Trans. Biomed. Eng. BME-15 pp. 128-129. 1968; and Ishijima, M., Shin, S. B., Hostetter, G. and Sklansky, J. Scan-along polygonal approximation for data compression of electrocardiograms, IEEE Trans. Biomed. Eng. BME-26 pp. 723-729. 1983.
The AZTEC algorithm has been used to store ECG data for automatic analysis functions such as QRS detection, but it has proven to be generally inadequate for a visual presentation of the data. The SAPA or fan compression method, on the other hand, uses a straight line approximation of the waveform to store the ECG data. This method does not generally permit substantial data compression. Since the error threshold is uniform over the entire input waveform, the allowable error in the P- and T-wave region limits the overall compression.
Yet another exemplary distorting method is the Mueller approach or turning point algorithm, as described in Mueller, W. C. Arrhythmia detection program for an ambulatory ECG monitor, Biomed. Sci. Instrum. 14: 81-85. 1978. The turning point algorithm provides a compression factor of two (2) by selecting one of every set of two data points in the waveform. The second of the two data points is generally selected unless the first data point is a turning point, in which case the first point is selected. While this technique generally preserves the fidelity of the system at low compression rates, it has not proven to be adequate when extended to high compression factors.
Wherefore, it would be highly desirable to have a new and improved method and apparatus for data compression in an ECG monitoring system having limited memory capacity. Such compression method and apparatus should provide a sufficiently high compression factor as well as clinically acceptable results from the reconstructed data, and should not require excessive computer processing time.